Floating gate transistors are well known in the state of the art. The gate of these transistors is “floating”, i.e. it is surrounded by an insulator. This means that the plates of the associated floating gate capacitor are electrically isolated. Therefore, a relatively high programming voltage is necessary to charge the floating gate capacitor by using a tunneling effect. The advantage of floating gate transistors is that a charge on the floating gate capacitor stays unchanged over years. Thus, a floating gate transistor may be programmed to be in a switched ON or in a switched OFF position without the need of energy for maintaining the programmed state.
Floating gate transistors may be used for example in trimming circuitries in transponders, especially in passive transponders where energy saving is essential. However, other applications are possible.
FIG. 1 shows a simplified schematic of such a trimming circuit using a floating gate transistor according to the state of the art. An inductance 12 and capacitors 14 and 16 form together a resonant circuit connected between a voltage VDD and ground. A capacitor 18 in series connection with the drain-source channel of a MOS field effect transistor 20 is connected in parallel to the series connection of capacitors 14 and 16. A capacitor 22 represents the floating gate capacitor of transistor 20. Two switches 24 and 26 allow connection of the plates of floating gate capacitor 22 either to ground or to a programming voltage VPP, i.e. a voltage sufficiently high to allow charges to tunnel to the plates. The plate of floating gate capacitor 22 which is connected to the gate of transistor 20 is coupled to switch 24 via protective anti-parallel zener diodes 28.
For trimming or adjusting the resonant frequency, capacitor 18 can be connected in parallel to the resonant circuit. Therefore, transistor 20 is used as a switch: if transistor 20 is conductive, capacitor 18 is connected in parallel, and if transistor 20 is not conductive, capacitor 18 is not switched in parallel.
For switching transistor 20, floating gate capacitor 22 is charged with different polarity. With switch 24 switched to ground and switch 26 switched to programming voltage VPP, floating gate transistor 20 is programmed ON. With switch 24 switched to programming voltage VPP and switch 26 switched to ground, floating gate transistor 20 is programmed OFF.
It is a disadvantage of floating gate transistors that a rather high programming voltage is needed for switching. A further disadvantage of floating gate transistors is that switching of a floating gate transistor requires several milliseconds.
In some applications a quick but short deactivation of the current state of a floating gate transistor, i.e. a short ON-OFF switching is desired. Examples are changing the resonant frequency of a resonant circuit to frequency modulate a signal or switching a parallel resonant circuit into a serial resonant circuit for example in oscillation maintenance circuits in passive half duplex transponders.
Additionally, in some of these applications energy consumption must be kept to an absolute minimum. Examples of these applications are passive half duplex transponders which have no battery at all or only a backup battery. Passive half duplex transponders get their energy out of a received high frequency signal and store this energy on a capacitor for use in sending a response signal. Thus, there is little energy available, and when the capacitor is discharged there is no supply voltage available at all.
There is a need for an electronic circuit comprising a floating gate transistor in which the programmed floating gate transistor can be deactivated temporarily even if there is no supply voltage available.